Mendel Concluded: Give Peas a Chance. But There's More... 3/3/11. by MC Doc W

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Imogene Higgins
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1 Mendel Concluded: Give Peas a Chance by MC Doc W There must be some sort of "particles" or "elements" inside the pea cells that make them look the way they do. Others had proposed that inheritance was a sort of "blending" of traits. But Mendel found no evidence for that. The trait of purple flower color is dominant to the trait of white flower color, which is said to be recessive because in a hybrid, the purple "element" somehow covers up the white "element". However, the white "elements" can be passed on although hidden, they may reappear in future generations. The same was true for the other six pairs of traits that Mendel studied... DOMINANT purple flowers round seeds yellow seeds inflated pods unripe pods green flowers along stems tall plant RECESSIVE white flowers wrinkled seeds green seeds constricted pods unripe pods yellow flowers at ends of stems dwarf plant But There's More... Mendel carefully counted the number of pea plants in each generation. When he crossed a true-breeding purple-flowered pea plant with a true-breeding white-flowered pea plant (P), all the offspring were purple-flowered (F 1 ). BUT... when he crossbred F 1 plants, his F 2 generation included 705 purple-flowered plants and 224 white-flowered plants, making 929 in all. This is very close to a ration of 3:1 three purpleflowered plants for every white-flowered plant. 1

2 3/3/11 Mendel observed the same thing in his other F2 crosses: there were always almost exactly three plants with the dominant trait for every one recessive (at least if you counted a large number of plants). DOMINANT RECESSIVE RATIO 5474 round seeds 1850 wrinkled seeds 2.96 : yellow seeds 2001 green seeds 3.01 : purple-flowered 224 white-flowered 3.15 : inflated pods 299 constricted pods 2.95 : green pods 152 yellow pods 2.82 : w/side flowers 207 w/end flowers 3.14 : tall plants 277 short plants 2.84 : 1 The white-flowered plants in the F2 generation only produced more white-flowered plants, for as long as Mendel kept raising more generations (F3, F4,....) It got even worse! Mendel looked more closely at the plants in the F2 generations, and tried letting them self-fertilize... One-third of the purple-flowered plants in the F2 generation were also true-breeding, producing nothing but more purple-flowered plants. 2

3 But two-third of the purple-flowered plants in the F 2 generation were not true-breeding they produced purpleflowered and white-flowered plants in the same 3:1 ratio! But two-third of the purple-flowered plants in the F 2 generation were not true-breeding they produced purpleflowered and white-flowered plants in the same 3:1 ratio! Mendel reasoned thus: There are "elements" inside the pea cells that determine what traits the peas have. We now call these genes. A gene may exist in several forms called alleles. In all the examples I've shown you, each gene has two alleles: purple / white flowers, green / yellow peas, etc. But a gene may (and often does) have one, three, four, or more alleles. More about that later! Mendel reasoned thus: Each pea plant has two copies of each of its genes. These two copies may be of the same allele, or each may be a different allele. When a plant's two copies of a gene are both the same allele, we call it homozygous. When a plant's two copies of a gene are different alleles, we call it heterozygous. 3

Mendel reasoned thus: An allele may be dominant or recessive. (There are a few other possibilities, but we'll look at them later; don't get sidetracked now!

Example: The F 1 pea plants had one dominant purple allele and one recessive white allele and all looked purple; the purple allele "masked" the white allele.

For example, we can call the dominant flower color allele P (purple), and the recessive flower color allele p (white) The alleles that any given plant is carrying can thus be represented by a pair of

4 Mendel reasoned thus: An allele may be dominant or recessive. (There are a few other possibilities, but we'll look at them later; don't get sidetracked now!) A dominant allele will cover up the existence of a recessive allele, if one is present. Example: The F 1 pea plants had one dominant purple allele and one recessive white allele and all looked purple; the purple allele "masked" the white allele. Dominant doesn t mean better or superior or anything like that. Mendel reasoned thus: We can represent dominant and recessive alleles of the same gene by capital and lower-case letters. For example, we can call the dominant flower color allele P (purple), and the recessive flower color allele p (white) The alleles that any given plant is carrying can thus be represented by a pair of letters. A whiteflowered plant must have the allele combination pp, but a purple-flowered plant could have the combination PP or Pp. Mendel's first crosses looked like this. Each of the F1 plants got one dominant (purple) allele from one parent, and one recessive (white) allele from the other parent. Here's what Mendel's crosses looked like in the second generation. Each F1 plant could pass on either the P allele or the p allele but not both. Mathematically, there are four possible outcomes. 4

To make this a little easier to visualize, we can use a calculating device invented after Mendel's death by G. H. Punnett, known as the Punnett square. Start by drawing a square, like so.

5 To make this a little easier to visualize, we can use a calculating device invented after Mendel's death by G. H. Punnett, known as the Punnett square. Start by drawing a square, like so... Now: Each plant contributes one of its pair of genes to each of its offspring. The purple-flowered pea plant can only pass on a purple allele... so you write in the alleles that that parent can pass on, at the top of the square. The whiteflowered pea plant can only pass on a white allele... so write in the alleles that that parent can pass on, at the left of the square. Now, fill in the square down and across. This gives you the possible genotypes of the offspring. 5

6 100% of the offspring get one P allele from one parent and one p allele from the other. In other words, the genotype of all the offspring is Pp. Since P is dominant to little p, the phenotype of all the offspring is "purple flowers." Now, watch how to set this up for the F 2 generation. Take two of the F 1 heterozygous purple-flowered peas, and crossbreed them. Each parent may contribute either a P allele or a p allele. 6

Now, fill in the square down and across. This gives you the possible genotypes of the offspring. Three out of four of the offspring will have the purple-flowered phenotype.

7 Now, fill in the square down and across. This gives you the possible genotypes of the offspring. Three out of four of the offspring will have the purple-flowered phenotype. One out of four will have the whiteflowered phenotype. This 3:1 ratio is what Mendel observed in his crosses. What's more, 1 out of 4 plants will be a homozygous or true-breeding purple (genotype PP); 2 out of 4 will be heterozygous purples (genotype Pp); and 1 out of 4 will be a homozygous white (genotype pp). This comes in handy when you start looking at crosses in which two pairs of genes are involved. Consider crossing a plant with yellow, smooth peas and a plant with wrinkled, green peas. 7

Mendel established that the allele for yellow peas is dominant to the allele for green peas, and the allele for round peas is dominant to the allele for wrinkled peas.

In this case, there is only one possibility: The yellow, smooth parent can only contribute the Y and S alleles, and the green, wrinkled parent can only contribute the y and s

8 Mendel established that the allele for yellow peas is dominant to the allele for green peas, and the allele for round peas is dominant to the allele for wrinkled peas. So here are the genotypes. Each parent contributes one of each pair of alleles that it has. In this case, there is only one possibility: The yellow, smooth parent can only contribute the Y and S alleles, and the green, wrinkled parent can only contribute the y and s alleles. This sets up a 4x4 Punnett square, but all of the F1 offspring have one Y and one S from one parent, and one y and one s from the other. All have the genotype YySs, and all have the phenotype "smooth yellow peas". But things get fun when you cross two of the F 1 plants! 8

9 Each parent contributes one of each pair of alleles that it has. Each parent can thus contribute any one of four possible allele combinations: YS, Ys, ys, or ys. Fill in the Punnett square down and across, and you get this... Now, if you determine the phenotypes, you get: 9 yellow smooth 3 yellow wrinkled 3 green smooth 1 green wrinkled out of every 16 offspring. 9

Trait: characteristics to be passed from parent to offspring. Passed on from parent to offspring by the blood, in bloodlines.

Inheritance Theory Prior to Mendel Trait: characteristics to be passed from parent to offspring Passed on from parent to offspring by the blood, in bloodlines. Gregor Mendel Father of Genetics Mendel s